Prior German patent application P 39 39 410 and corresponding International Application PCT/DE 90/00862, now U.S. Ser. No. 07/839,757, filed Apr. 10, 1992, assigned to the assignee of the present application, disclose a thermo-sensitive acceleration and tilt sensor adapted for attachment to a movable object, particularly a motor vehicle.
A housing defines a rotation-symmetrical interior space or cavity which tapers down vertically in a conical shape. The space is filled by fluid up to an air bubble. On the cover plate of the inside of the housing, an ohmic resistance element is applied, which forms a part of an evaluation circuit of the sensor, in the form of a Wheatstone bridge circuit. The construction of the electrical element using thick-film technology, to make an electrically heatable and temperature-sensitive element, is described.
The mode of operation of the sensor is the heat-dissipation-mediated change in resistance of the ohmic resistance element, which is heated to a specified temperature.
In the rest position, the circuit element is in thermal contact with the air bubble in the interior space of the sensor. If the sensor is accelerated and/or tilted, the air bubble moves, and the fluid flows over the circuit element. The change in heat dissipation relationships lead to a change in the ohmic resistance of the circuit element, which in turn is detected by the evaluation circuit. This sensor is particularly well adapted for threshold value detection of tilt and acceleration of a movable object.